1. Field of the Invention
The present invention relates to an optical pickup device and an optical disc device, and more particularly to a technology adapted to record/reproduce with respect to a recording medium having laminated recording layers.
2. Disclosure of Related Art
In recent years, as the capacity of an optical disc has been increased, an optical disc having an increased number of recording layers has been developed. Laminating recording layers in a disc enables to considerably increase the data capacity of the disc. In the case where recording layers are laminated, generally, two recording layers are laminated on one side of a disc. Recently, however, laminating three or more recording layers on one side of a disc has been put into practice to further increase the capacity of the disc. Thus, the capacity of a disc can be increased by increasing the number of recording layers to be laminated. However, as the number of recording layers to be laminated is increased, the distance between the recording layers is decreased, and signal deterioration resulting from an interlayer crosstalk is increased.
As the number of recording layers to be laminated is increased, reflection light from a recording layer (a targeted recording layer) to be recorded/reproduced is reduced. As a result, if unwanted reflection light (stray light) is entered into a photodetector from a recording layer on or under the targeted recording layer, a detection signal may be deteriorated, which may adversely affect focus servo control and tracking servo control. In view of this, in the case where a large number of recording layers are laminated, it is necessary to properly remove stray light, and stabilize a signal from a photodetector.
As a method for removing stray light, there is proposed a method using a pinhole. In this method, a pinhole is formed at a position where signal light is converged. In this method, an unwanted stray light component entered into a photodetector can be reduced, because a part of stray light is blocked by the pinhole. There is proposed a method using a half wavelength plate and a polarizing optical element in combination, as another method for removing stray light. In this method, a polarization direction of stray light is changed by the half wavelength plate, and the stray light is blocked by the polarizing optical element. This enables to prevent an unwanted stray light component from being entered into a photodetector.
However, in the method for removing stray light using a pinhole, it is necessary to accurately position the pinhole at a position where laser light (signal light) reflected on a targeted recording layer is converged. In this method, therefore, it is difficult to adjust the position of the pinhole. If the size of the pinhole is increased to easily adjust the position of the pinhole, stray light is more likely to pass through the pinhole, which obstructs the effect of suppressing signal deterioration resulting from stray light.
In the method for removing stray light by combined use of a half wavelength plate and a polarizing optical element, each two half wavelength plates and polarizing optical elements are necessary. In addition, two lenses are necessary to remove stray light. Thus, the number of parts and the cost are increased. Further, it is cumbersome to adjust the arrangement positions of these members. Furthermore, it is necessary to secure a space for arranging these members side by side, which may increase the size of an optical system.
In the case where a recording/reproducing operation is performed with respect to an optical disc of the above type, a servo signal such as a focus error signal and a tracking error signal is generated, in addition to a reproduction RF signal, based on current signals to be outputted from a photodetector. The photodetector includes a plurality of sensors divided each other so that a servo signal can be generated. Current signals to be outputted from the respective sensors are subjected to amplification and voltage conversion by I/V amplifiers. A reproduction RF signal and a servo signal are generated by summation and subtraction of voltage signals obtained by the above process.
However, an inherent noise is generated in the I/V amplifiers at the time of I/V conversion. The noise may be superimposed on voltage signals from the respective sensors. As a result, a reproduction RF signal and a servo signal may be deteriorated resulting from the noise.
In particular, since a reproduction RF signal is generated by summation of all the voltage signals from the sensors, all the noises generated in the I/V amplifiers may be superimposed on the reproduction RF signal. On the other hand, since the frequency of a reproduction RF signal is high, as compared with a servo signal, deterioration of an electrical characteristic of the reproduction RF signal is large, as compared with the servo signal. The deterioration becomes conspicuous, as the recording/reproducing speed is increased. If a noise resulting from an I/V amplifier is superimposed on a reproduction RF signal whose electrical characteristic is likely to be deteriorated, the S/N ratio of the reproduction RF signal may be seriously lowered. If the deterioration of the reproduction RF signal progresses, it may be impossible or difficult to compensate for the deterioration by an error correction circuit provided posterior to the I/V amplifiers. In view of the above, it is necessary to provide a measure for suppressing noise resulting from the I/V amplifiers particularly with respect to a reproduction RF signal.